1. Field of the Invention
The invention relates to a multi-layered printed circuit board including two or more layers therein, used for an electronic device such as a data processing apparatus and a communication apparatus, and more particularly to such a printed circuit board capable of preventing unintentional electromagnetic interference and also depressing circuit malfunction caused by electromagnetic noises intruding thereto from outside.
2. Description of the Related Art
With recent development of network system, various problems are posed such as unintentional electromagnetic interference (EMI) generated by operation of electronic devices such as a data processing device and communication device, interference to other radio-communication systems by such EMI, and malfunction of electronic devices caused by intrusion of external electromagnetic waves thereinto.
In order to overcome those problems, electronic devices are sealed, for instance, with a filter for preventing electromagnetic interference from emitting therefrom and external electromagnetic waves from intruding thereinto. To seal a printed circuit board on which an electronic circuit, a major part of an electronic device, is mounted brings an advantage in terms of a cost, and hence various attempts have been made for that purpose.
For instance, one of conventional multi-layered printed circuit boards has been suggested in Japanese Unexamined Patent Publication No. 5-243782. FIG. 1 is a cross-sectional view of the suggested multi-layered printed circuit board. A multi-layered printed circuit board 101 includes a signal transmission layer 102, an internal earth layer 104 located above the signal transmission layer 102, an earth layer 105 located above the signal transmission layer 102, and an insulator 103 filled between an upper surface layer 100 and the earth layer 105. An amplifier circuit 99 is mounted on the upper surface layer 100.
Japanese Unexamined Utility Model Publication No. 60-71174 has suggested another multi-layered printed circuit board. As illustrated in FIG. 2, the suggested multi-layered printed circuit board 106 includes power-supplying lines 109 and 110 connected to a power-supply, a plurality of insulating substrates 108 deposited one on another between the power-supplying lines 109 and 110, and signal layers 107 (shown with a broken line) electrically connecting elements, mounted on the printed circuit board 106, with each other. The signal layers 107 are formed in the insulating substrates 107 to thereby isolate the signal layers 107 from circumstances.
FIG. 3 illustrates a multi-layered printed circuit board suggested in Japanese Unexamined Utility Model Publication No. 5-76083. The suggested printed circuit board 111 includes a pair of outer surface layers 140, and signal pattern layers 141a, 141b, 141c and 141d formed between the outer surface layers 140. The outer surface layers 140 are designed to have a ground pattern or a solid pattern in order to act as a shield, thereby preventing emission of electromagnetic waves from the signal pattern layers 141a to 141d.
FIG. 4 illustrates a multi-layered printed circuit board suggested in Japanese Unexamined Patent Publication No. 2-68571. The suggested printed circuit board 112 includes a first insulating layer 142, a first signal pattern layer 143, a second insulating layer 144, a second signal pattern layer 145, a substrate 146, and a shield layer 113, deposited in this order. A device 147 is mounted on the shield layer 113, but is designed to make no electrical communication with the shield layer 113.
FIG. 5 illustrates a multi-layered printed circuit board suggested in Japanese Unexamined Utility Model Publication No. 59-146999. In accordance with the suggested printed circuit board 114, signal layers 116 are sandwiched between ground or power-supplying layers 115.
FIG. 6 illustrates a structure for packaging an electronic device therein suggested in Japanese Unexamined Utility Model Publication No. 2-26294. A transformer 148 comprising a core 149 and a coil 150 wound around the core 149 is placed in a case 151. On the transformer 148 is mounted a multi-layered printed circuit board 121. The multi-layered printed circuit board 121 is designed to have a lowermost layer as a shield layer 122. Since the shield layer 122 is located closest to the core 149 or a source of emission of electromagnetic waves, the shield layer 122 shuts out noises derived from the core 149.
FIG. 7 illustrates a structure for packaging an electronic device therein suggested in Japanese Unexamined Patent Publication No. 6-69680. A multi-layered printed circuit board 124 is designed to have outer surface layers as shield layers 125a and 125b. The multi-layered printed circuit board 124 is mounted in a metal box 126 to thereby define a space between the printed circuit board 124 and a bottom surface of the metal box 126. A source 127a of emission of high frequency electromagnetic waves and a circuit 127b are disposed in the space to thereby shield EMI source.
FIG. 8 is a top plan view illustrating a printed circuit board suggested in Japanese Unexamined Patent Publication No. 7-302956. The illustrated printed circuit board has a substrate 128 on which conductors 130A and 130B are formed at a marginal end thereof, both of which define a rectangular-shaped conductor. A plurality of conductors 130D extend in a lengthwise direction of the rectangular conductor 130A and 130B, and a plurality of intermediate conductors 130E extend in the same direction between the conductors 130D. On the substrate 128 are formed an oscillator 152, and integrated circuits 153a and 153b. The oscillator 152 and the integrated circuits 153a, 153b are electrically connected to one another by a clock line 154. The substrate 128 is formed with a rectangular opening 129 having a width W.sub.1 and located at a distance L.sub.4 from the conductor 130A. The rectangular opening 129 divides the intermediate conductors 130E into pieces, and hence define a non-conductive region. The rectangular opening 129 is located remote from the conductors 130A and 130B by a distance of one-fourth or greater of lengths of the conductors 130A and 130B, respectively.
FIG. 9 illustrates a multi-layered printed circuit board suggested by Y Akiba, Proceedings of the 10th Japan Institute for Interconnecting and Packaging Electronic Circuits Annual Meeting, 15B-12, pp. 175-176. The illustrated printed circuit board 131 includes first and second ground layers 132 and 133 between which is interposed a resistor 135. A power-supplying layer 160 is sandwiched between the ground layers 132 and 133. A dielectric substance 161 disposed between the ground layers 132 and 133 is designed to have a dielectric constant .epsilon.1 greater than .epsilon.2 of other dielectric substance 162 disposed between the power-supplying layer 3 and the ground layer 133 in order to obtain a great capacity between the power-supplying layer 3 and the ground layer 132.
There are two major factors for unintentional electromagnetic interference from a printed circuit board and malfunction of a circuit caused by intrusion of external electromagnetic field thereinto. One of them is a loop defined by a signal wiring pattern and a ground pattern, and the other is fluctuation in voltage between a power-supply and ground.
The printed circuit boards illustrated in FIGS. 1 to 5 attempt to prevent unintentional electromagnetic interference and intrusion of external electromagnetic field both caused by the firstly mentioned factor, namely a loop defined by a signal wiring pattern and a ground pattern. Those printed circuit boards attempt to electrically shield a loop defined by a signal wiring pattern and a ground pattern by the ground layer formed all over a substrate, the ground pattern partially formed on a substrate, or the signal wiring pattern sandwiched between metal plates having the same potential as that of a power-supplying layer and a power-supplying pattern layer. However, those printed circuit boards have a problem that layers having the same potential as that of ground or a power-supply have to be additionally formed for sandwiching a signal wiring pattern layer therebetween.
The printed circuit board illustrated in FIGS. 6 and 7 are designed to have a layer or layers as an entire ground layer or as a shield plate to thereby define a space between the shield plate and a metal housing. A high frequency circuit which is a source of EMI is placed in the space to thereby prevent EMI therefrom and intrusion of external electromagnetic waves thereinto.
However, in the above-mentioned structure where a layer or layers of the printed circuit board cooperate(s) with a housing to thereby define a shield, the printed circuit board has to be fabricated so as to be able to assemble with a particular housing. Hence, the printed circuit board one fabricated to do so cannot be applied to existing housings.
The printed circuit board illustrated in FIG. 8 divides the power-supplying pattern and ground pattern into pieces by means of the intermediate non-conductors 129 to thereby prevent resonance caused in dependence on a length of those patterns. However, if the power-supplying pattern or a ground pattern were designed to entirely cover a substrate therewith, the non-conductors 129 inversely act as a slit antenna, resulting in increased EMI.
The multi-layered printed circuit board illustrated in FIG. 9 incorporates the resistor 135 as loss material into a high frequency circuit defined by the first and second ground layers 132, 133 and the power-supplying layer 160 to thereby prevent EMI and malfunction of devices both caused by fluctuation in a voltage between the power-supplying layer 160 and the first and second ground layers 132, 133. However, the printed circuit board has a problem that an additional ground layer has to be formed other than an ordinary ground layer, resulting in an increase in fabrication cost.